Split Images of Quasars Help us Better Understand Expansion of the Universe

January 23, 2019
4 minutes read
Split Images of Quasars Help us Better Understand Expansion of the Universe

Knowing the velocity of the expansion of the universe is still puzzling to astronomers. But a team of astronomers has taken a step toward resolving the debate.

By studying quasars, fountains of radiation powered by black holes, researchers from the University of California, Los Angeles have found a way to measure the rate in which the universe is expanding.

They published their research yesterday in Monthly Notices of the Royal Astronomical Society.

Scientists are now using split images of quasars to produce an estimate of the Hubble Constant or the measurement used to describe the expansion of the universe.

The Hubble constant is a number that relates distances to the redshifts of galaxies—the amount that light stretches as it travels to Earth through the expanding universe. Previous estimates for the Hubble constant range from about 67 to 73 kilometers per second per megaparsec. Therefore, two points in space 1 megaparsec apart (the equivalent of 3.26 million light-years) are racing away from each other at a speed between 67 and 73 kilometers per second.

“The Hubble constant anchors the physical scale of the universe,” said Simon Birrer, a UCLA postdoctoral scholar and lead author of the study. Without a precise value for the Hubble constant, astronomers can’t accurately determine the sizes of remote galaxies, the age of the universe or the expansion history of the cosmos.

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However, the researchers proposed a new ingredient that scientists have not used in their calculations to infer the Hubble Constant. In their study, they chose quasars whose light has been bent by the pull of an intervening galaxy. Thus, producing two side-by-side images.

The light between these two images takes different routes to Earth. When the quasar’s brightness fluctuates, the two images flicker one after another, rather than at the same time. Astronomers can use the delay in time between those two flickers, along with information about the meddling galaxy’s gravitational field, to trace the light’s journey and deduce the distances from Earth to both the quasar and the foreground galaxy. Knowing the redshifts of the quasar and galaxy has enabled the scientists to estimate how quickly the universe is expanding.

To demonstrate this technique, they observed the split images of the quasar SDSS J1206+4332. They used the data collected by the Hubble Space Telescope, the Gemini Observatory, W.M. Keck Observatory, and the Cosmological Monitoring of Gravitational Lenses (COSMOGRAIL) network. They took photos of the quasar every day for several years to measure the time delay between the split images.

Then, to get the best estimate possible of the Hubble constant, they combined the data they have gathered on that quasar with data gathered by their H0liCOW collaboration on three quadruply imaged quasars.

However, the researchers are still looking for more quasars to improve the precision of the technique. The team is looking at 40 quadruply image quasars to see what else they can learn.

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Thumbnail image: Image from the Hubble Space Telescope of a doubly imaged quasar. Credit: NASA Hubble Space Telescope, Tommaso Treu/UCLA, and Birrer et al.

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